On 16.08.21 16:40, Matthew Wilcox wrote:

On Mon, Aug 16, 2021 at 04:33:09PM +0200, David Hildenbrand wrote:

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I did not follow why we have to play games with MAP_PRIVATE, and having
private anonymous pages shared between processes that don't COW, introducing
new syscalls etc.

It's not about SHMEM, it's about file-backed pages on regular
filesystems.  I don't want to have XFS, ext4 and btrfs all with their
own implementations of ARCH_WANT_HUGE_PMD_SHARE.

Let me ask this way: why do we have to play such games with MAP_PRIVATE?

Are you referring to this?

Yes

: Mappings within this address range behave as if they were shared
: between threads, so a write to a MAP_PRIVATE mapping will create a
: page which is shared between all the sharers.

If so, that's a misunderstanding, because there are no games being played.
What Khalid's saying there is that because the page tables are already
shared for that range of address space, the COW of a MAP_PRIVATE will
create a new page, but that page will be shared between all the sharers.
The second write to a MAP_PRIVATE page (by any of the sharers) will not
create a COW situation.  Just like if all the sharers were threads of
the same process.

It actually seems to be just like I understood it. We'll have multiple 
processes share anonymous pages writable, even though they are not using 
shared memory.

IMHO, sharing page tables to optimize for something kernel-internal 
(page table consumption) should be completely transparent to user space. 
Just like ARCH_WANT_HUGE_PMD_SHARE currently is unless I am missing 
something important.

The VM_MAYSHARE check in want_pmd_share()->vma_shareable() makes me 
assume that we really only optimize for MAP_SHARED right now, never for 
MAP_PRIVATE.

-- 
Thanks,

David / dhildenb